O descendant of Bharata, he who dwells in the body is eternal and can never be slain. Therefore you need not grieve for any creature.

PURPORT

The Lord now concludes the chapter of instruction on the immutable spirit soul. In describing the immortal soul in various ways, Lord Krsna establishes that the soul is immortal and the body is temporary. Therefore Arjuna as a ksatriya should not abandon his duty out of fear that his grandfather and teacher—Bhisma and Drona—will die in the battle. On the authority of Sri Krsna, one has to believe that there is a soul different from the material body, not that there is no such thing as soul, or that living symptoms develop at a certain stage of material maturity resulting from the interaction of chemicals. Though the soul is immortal, violence is not encouraged, but at the time of war it is not discouraged when there is actual need for it. That need must be justified in terms of the sanction of the Lord, and not capriciously.